1. Field of the Invention
The present invention relates to an apparatus for applying a coating layer of a thermoplastic resin on the outer periphery of a linear body; and, in particular, to a structure of the apparatus capable of precisely producing a cross-sectional form of the coating layer.
2. Related Background Art
FIG. 1 is a sectional view of a slotted rod. This slotted rod B comprises a rod core A, which is constituted by a tension member 1 disposed at its center and a primary coating layer 2 disposed on the outer periphery of the tension member, and a coating layer 4 (spacer) disposed on the outer periphery of the rod core A. The surface of the coating layer 4 is provided with slots 3 (grooves for accommodating optical fibers) spirally extending along the longitudinal direction of slotted rod B. An optical cable in which an optical fiber ribbon (including a plurality of optical fibers) is inserted in each slot 3 has excellent characteristics in that it can attain higher density in containing fibers, workability would improve since connection can be made ribbon by ribbon, and so forth. In order for each of the optical fiber ribbons to maintain its transmission characteristics, the slotted rod B is required to be made with high accuracy. Therefore, for example, Japanese Utility Model Application Laid-Open No. 10-101211 or Japanese Patent Application Laid-Open No. 61-179408 discloses a method of making a slotted rod by a resin extrusion coating apparatus.
FIG. 2 is a schematic view showing a configuration of such a conventional extrusion coating apparatus for making a linear body having a coating layer such as slotted rod B. In this drawing, the tension member 1 paid out from a drum 53 is guided via a guiding device 50a to a first coating unit 20a attached to the front end portion of a first extrusion unit 10a, where the primary coating 2 made of adhesive polyethylene is applied thereto, thereby the rod core A having a circular cross section is formed.
As the rod core A is passed through a cooling bath 51a, the primary coating 2 is cured. Thereafter, the rod core A passed through the cooling bath 51a is sent to a second coating unit 20b attached to a second extrusion unit 10b, where the spacer 4 having the spiral slots 3 therearound is applied thereto, thereby the slotted rod B is obtained. Thus obtained slotted rod B is further introduced into a cooling bath 51b so as to be cooled and solidified and then is wound up around a take-up drum 54 by way of a guiding device 50b and a capstan 52.
Having studied the conventional extrusion coating apparatus, the inventors have found the following problems. Namely, in the conventional extrusion coating apparatus, the tension member 1 or rod core A is set by the guiding devices 50a, 50b such as to run at a predetermined position constantly. On the other hand, the first coating unit 20a and the second coating unit 20b are fixed at the front end portions of their corresponding extrusion units 10a and 10b, respectively. As a consequence, if the front end portion of the extrusion unit 10a, 10b moves upon thermal expansion to a direction (Y-axis direction) orthogonal to the drawing surface, the disposed position of the first or second coating unit 20a, 20b will fluctuate, thereby the relative positions of the first coating unit 20a and tension member 1 or the relative positions of the second coating unit 20b and rod core A will change with respect to each other. Hence, in thus obtained rod core A, the tension member 1 may become eccentric with respect to the center of the primary coating 2, or the rod core A may become eccentric with respect to the center of the spacer 4. When the rod core A becomes eccentric excessively with respect to the center of the spacer 4, bumps and dips may be generated at the bottom of the spiral slots 3 because of loss of resin at the bottom of the spiral slots 3, thereby problems may occur in that the optical fiber ribbons inserted in the slots 3 are damaged by the bumps and dips.
Also, since each coating unit is fixed at the front end portion of its corresponding extrusion unit in the conventional extrusion coating apparatus, there have been problems in that the vibration of the coating unit would increase when a die therein is rotated at a high speed in one direction or alternately in opposite directions, and so forth.
Therefore, it is an object of the present invention to provide an extrusion coating apparatus having a structure which is adapted to overcome the above-mentioned problems, thereby being capable of forming and applying a coating layer with a predetermined cross-sectional form onto the outer periphery of a linear body, such as a tension member and a rod core, as a target linear body, at a high speed even when thermal distortion occurs in its extrusion unit.
The extrusion coating apparatus according to the present invention comprises, at least, an extrusion unit extruding a thermoplastic resin while heating the resin, and a coating unit applying the extruded resin on the outer periphery of a linear body in a predetermined form while the linear body is passing therethrough. The extrusion unit and coating unit are connected to each other with a connecting tube guiding the heated thermoplastic resin from the extrusion unit to the coating unit. The connecting tube has a first opening connected to a resin inlet of the coating unit; and a second opening, opposing to the first opening, connected to a resin-extruding orifice of the extrusion unit.
In order to achieve the above-mentioned object, the extrusion coating apparatus according to the present invention further comprises a distortion-absorbing structure absorbing the distortion generated in the extrusion unit upon heating the thermoplastic resin.
For example, the distortion-absorbing structure can be realized by a rail extending along a predetermined direction with respect to the coating unit, and a pedestal movable on the rail in a state carrying the extrusion unit.
Thus, the coating unit is fixedly installed on a horizontal basis (corresponding to an installation floor surface), whereas the extrusion unit is movably disposed on the horizontal basis (by the distortion-absorbing structure). As a consequence, even when the extrusion unit is heated and thereby thermal distortion occurs to cause a positional fluctuation of the extrusion unit, the relative positional relationship between the coating unit and the linear body running therethrough would not change. Since the relative positions of the linear body and the coating unit with respect to each other are thus always kept constant, a coating layer having a predetermined form can be applied to the linear body without unevenness. Also, the vibration can be minimized in a coating unit having a rotator.
The installed positions of the extrusion unit and coating unit may have various modes. For example, the extrusion unit may be disposed such that the pass-line direction of the linear body and the direction of the heated thermoplastic resin extrusion are orthogonal to each other. Preferably, in this case, the connecting tube has such a form that straightly extends from the first opening toward the second opening while having a side face continuous from the first opening to the second opening.
In thus configured extrusion coating apparatus, even when the extrusion unit is heated and then this heat is conducted to the connecting tube to cause thermal expansion, the resulting thermal distortion is canceled as the extrusion unit moves about the coating unit. Therefore, the relative positions of the linear body and the coating unit are always kept constant with respect to each other, thereby a coating layer having a predetermined form can be applied to the linear body without unevenness. Also, since the main body of the extrusion unit and the coating unit have structures substantially identical to those of the conventional apparatus, they can be embodied easily.
The extrusion unit may also be disposed such that an angle formed between the pass-line direction of the pulled linear body and the extruded direction of the heated thermoplastic resin is not less than 0xc2x0 and is less than 90xc2x0. In this case, the connecting tube has such a form that is bent at a predetermined location from the first opening toward the second opening while having a side face continuous from the first opening to the second opening.
In thus configured extrusion coating apparatus, when the extrusion unit is heated, this heat is conducted to the connecting tube. As a consequence, thermal distortions occur in the extrusion unit mainly along the extruding direction thereof, and in the L-shaped connecting tube along the extruding direction and the direction orthogonal thereto. These thermal distortions, however, are canceled as the extrusion unit moves about the coating unit with the aid of the connecting tube. As a consequence, the relative positions of the linear body and coating unit with respect to each other are always kept constant, thereby the coating layer having a predetermined form can be applied to the linear body without unevenness. Also, when the pass-line direction of the linear body and the extruding direction of the extrusion unit are disposed in parallel to each other, then a plurality of extrusion coating apparatus can be arranged close to each other, thereby the floor area can efficiently be used.
The distortion-absorbing structure can also be realized by changing the configuration of the connecting tube. Namely, the connecting tube may be provided with the distortion-absorbing structure which comprises a first flexible joint section including the first opening and a second flexible joint section including the second opening.
As a consequence, even when each of the coating unit and the extrusion unit is fixedly installed, they are connected to each other with the connecting tube having the distortion-absorbing structure, thereby the positional fluctuation of the extrusion unit caused by thermal distortion would not influence the relative positions of the linear body and the coating unit with respect to each other. Therefore, a coating layer having a preferable cross-sectional form can be formed on the outer periphery of the linear body.
In the case where the extrusion coating apparatus according to the present invention is employed as the apparatus for making a slotted rod such as that mentioned above, the extrusion coating apparatus can provide a primary coating layer around a tension member to form a rod core and then the spacer around a rod core. In this case, the coating unit includes a rotatable die having a through hole through which the linear body penetrates, the rotatable die having a protrusion extending from the inner wall of the through hole toward the center of the through hole. In this coating unit, as the rotatable die is rotated in a predetermined direction about the linear body or the rotating direction of the rotatable die is reversed every time the linear body travels a predetermined distance, a spiral slot is formed on the coating layer surface of the linear body. Here, the die in the coating unit is not necessarily be rotated about the linear body but may also be made stationary.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.